Compressed natural gas (CNG) is a high octane fuel that is beneficial for reducing engine knock, for reducing hydrocarbon emissions in cold start events, and for reducing carbon dioxide emissions during engine operations. However, CNG has a low energy density compared to liquid fuels, such as diesel fuel or gasoline. This typically requires packaging of CNG in cryogenic quality cylinders (as liquified natural gas (LNG)) or in high pressure tanks (approximately 200-250 atmospheres).
To increase the range and total fuel quantity stored in a vehicle, CNG may be utilized in conjunction with gasoline or diesel fuel, requiring the vehicle to switch between fuels for optimal performance. However, space constraints do not allow for the inclusion of separate fuel tanks to all vehicles. A preferable system may be one that stores liquid fuel and pressurized gaseous fuel together in a single tank. In particular, CNG is able to partially dissolve in gasoline or diesel fuel when stored together at relatively low pressure (˜100 atm).
Storing a mix of pressurized gaseous fuel and liquid fuel within a single tank presents challenges for fuel usage in order to gain the benefits of having both pressurized gaseous fuel and liquid fuel available for combustion. While fuel usage strategies have been developed for vehicles with multiple fuel tanks, these solutions are insufficient to control fuel usage for a vehicle with a single, mixed fuel tank. In some examples, liquid fuel operations may dependent on a fuel tank pressure being above a threshold in order to distribute liquid fuel to a fuel rail. Further, the liquid phase component may contain a mixture of CNG and gasoline or diesel fuel, for example. This mixture has different properties than either CNG or gasoline/diesel fuel alone, and must be accounted for in the vehicle's fuel usage strategy.
The inventors herein have recognized the above problems, and have developed systems and methods to at least partially address these problems. In one example a method for an engine, comprising: responsive to a pressure in a fuel tank being below a pressure threshold, injecting only a liquid fuel into an engine cylinder, the fuel tank storing the liquid fuel and a pressurized gaseous fuel partially dissolved in the liquid fuel. In this way, the pressurized gaseous fuel may be conserved, thus maintaining a pressure gradient within the fuel system and allowing for judicious use of the pressurized gaseous fuel, for example during cold start conditions.
In another example, a fuel system for an internal combustion engine, comprising: a fuel tank configured to store a liquid fuel and a pressurized gaseous fuel capable of partially dissolving in the liquid fuel; a group of direct fuel injectors in communication with a group of cylinders; a first fuel line coupled between the group of direct fuel injectors and the fuel tank, the first fuel line configured to supply liquid fuel to the group of direct fuel injectors; a group of port fuel injectors in communication with the group of cylinders; a second fuel line coupled between the group of port fuel injectors and the fuel tank, the second fuel line configured to supply pressurized gaseous fuel to the group of port fuel injectors; and a controller configured with instructions stored in non-transitory memory and executable by a processor to: responsive to a pressure in the fuel tank being below a pressure threshold, operating the group of cylinders with fuel from the group of direct fuel injectors, and not with fuel from the group of port fuel injectors. In this way, the engine may operate solely on direct-injected liquid fuel while maintaining sufficient octane availability
In yet another example, a method for an engine having a fuel tank configured to store a liquid fuel and a pressurized gaseous fuel capable of partially dissolving in the liquid fuel, comprising: responsive to a pressure in the fuel tank being below a pressure threshold, injecting only a liquid fuel into an engine cylinder; and responsive to a liquid level in the fuel tank being below a liquid level threshold, injecting only the pressurized gaseous fuel into the engine cylinder. In this way, the ability of the engine to operate on liquid fuel, pressurized gaseous fuel, or a combination of both fuels as necessary may be maintained based on current fuel availability.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.